1. Field of the Invention
The present invention relates to a filter apparatus having a plurality of resonators, a duplexer, and a communication apparatus, such as a base station communication apparatus.
2. Description of the Related Art
In the related art, resonators used in the microwave band and capable of handling relatively large power include a cavity resonator and a semi-coaxial resonator. A semi-coaxial resonator is also known as a coaxial cavity resonator, and is relatively useful to form a compact filter etc. because of its relatively high Q factor and because it is more compact than a cavity resonator.
FIG. 11 is a top view of a filter including semi-coaxial resonators, with a cavity lid removed. A cavity body 1 having an opening, which is covered by a cavity lid, includes cylindrical conductive bars 4 in the centers of the cavities of the resonators in order to form a plurality of semi-coaxial resonators. Adjacent resonators are coupled to each other by known arrangements.
A filter having TM dual-mode dielectric resonators may also be useful to provide a compact resonator.
FIG. 12 shows an example of a filter using TM dual-mode dielectric resonators. In FIG. 12, a cavity body 1 includes a cruciform dielectric core 3 in each resonator space so as to provide multiplexing of the two perpendicular TM modes.
With the advent of micro-cell cellular mobile communication systems such as cellular phones, the demand for more compact filters in base stations has increased. In addition, as the number of installed filters has increased, more cost-effective filters have been increasingly required.
However, a filter having semi-coaxial resonators still requires a large volume for each resonator, and thus the overall filter apparatus cannot be reduced in size. A filter apparatus having TM dual-mode resonators includes resonators formed of dielectric cores in all stages, and therefore may be compact as a whole; however, it requires a complicated manufacturing process for integral molding, thereby making it difficult to achieve cost-effectiveness.
Accordingly, the present invention addresses the above problems by providing a compact and low-cost filter apparatus, a duplexer, and a communication apparatus incorporating these features.
To this end, in a first aspect of the present invention, a filter apparatus includes a dual-mode resonator and a TEM single-mode resonator. The dual-mode resonator includes a conductive cavity that houses a conductive bar having at least one end electrically connected to the cavity and a dielectric core through which the bar is inserted. The dual-mode resonator duplexes and couples a TEM mode generated by the cavity and the bar and a TM mode generated by the cavity and the dielectric core. The TEM single-mode resonator includes a conductive cavity which houses a conductive bar having at least one end electrically connected to the cavity.
A dual-mode, i.e., both TEM-mode and TM-mode, resonator may be used to achieve a compact filter apparatus. In addition, the dual-mode resonator is combined with a TEM single-mode resonator to construct a filter apparatus having a predetermined number of stages of resonators within a limited space at low cost.
In another aspect of the present invention, a duplexer includes a reception filter and a transmission filter. The reception filter includes a plurality of dual-mode resonators as described above, wherein predetermined resonators between adjacent dual-mode resonators are coupled with each other. The transmission filter includes a dual-mode resonator and a TEM single-mode resonator, wherein predetermined resonators between adjacent resonators are coupled with each other. The duplexer further includes a shared input/output port which provides an input to the reception filter and an output from the transmission filter.
A reception filter which generally requires a greater number of stages of resonators than a transmission filter is formed of a plurality of dual-mode resonators, and can therefore be reduced in size. A transmission filter includes a dual-mode resonator and a TEM single-mode resonator, and can thus provide the same resonator length in the alignment direction as that in the reception filter, while satisfying required frequency characteristics. Accordingly, a duplexer having such a reception filter and transmission filter can be made compact, in which the lengths of the resonators in the reception and transmission filters can be uniform in an alignment direction of the resonators. The duplexer can therefore be readily assembled into a communication device.
The duplexer may further include a low-noise amplifier circuit for amplifying a reception signal output from the reception filter, wherein the low-noise amplifier circuit, the transmission filter, and the reception filter are housed by a housing. This provides a shorter distance from the reception filter to the low-noise amplifier circuit, thereby suppressing incoming noise, so that a reception signal having a high signal-to-noise ratio can be output from the duplexer.
The duplexer may further include a low-pass filter between the shared input/output port and an antenna port, for transmitting a signal component in the transmission and reception frequency bands, and blocking a signal component in the frequency regions higher than the transmission and reception frequency bands. This can suppress emission of unwanted signals due to spurious modes.
In still another aspect of the present invention, a communication apparatus, such as a base station communication apparatus, includes the aforementioned duplexer, and a transmitter and a receiver which are connected to the duplexer. A base station communication apparatus, for example, can thus be made compact and cost-effective.